The invention relates generally to semiconductor device and integrated circuit fabrication and, in particular, to structures for a cascode integrated circuit, structures for a field-effect transistor, and methods of forming such structures.
Radio-frequency integrated circuits (RFICs) are found in many type of devices, such as mobile phones. An RFIC generally requires a switch to select and control connections between an antenna, a transmitter circuit (e.g., a power amplifier), and a receiver circuit (e.g., a low noise amplifier or LNA). A main purpose of an LNA is to provide gain that boosts the power of received signals while preserving the input signal-to-noise ratio at output, which is an important characteristic because the received signals are usually weak and can be commingled with a large amount of noise.
Most conventional LNAs are constructed either with a single transistor configuration like a common emitter or common source or with a cascode design. A cascode circuit is often constructed from two transistors, either a pair of bipolar junction transistors or a pair of field effect transistors, with one of the transistors operating as a common emitter or common source and the other of the transistors operating as a common base or common gate. A cascode circuit constructed from only field-effect transistors may cause the associated LNA to suffer from low input intercept point (IIP3) and low gain issues on the input power axis. A cascode circuit constructed from only bipolar junction transistors may cause the associated LNA to be incapable of operating at a low voltage due to an insufficient voltage across the collector-emitter junction (Vce).
Improved structures for a cascode integrated circuit, structures for a field-effect transistor, and methods of forming such structures are needed that overcome these and other deficiencies of conventional cascode integrated circuits.